artists: add is_deleted and is_banned indexes.

db/migrate/20200307021204_add_is_deleted_index_on_artists.rb

@@ -0,0 +1,6 @@
+class AddIsDeletedIndexOnArtists < ActiveRecord::Migration[6.0]
+  def change
+    add_index :artists, :is_deleted
+    add_index :artists, :is_banned
+  end
+end